Exemplary embodiments pertain to reducing costs and simplifying manufacturing of stator windings of electric machines and, more particularly, to achieving high machine efficiency and high manufacturing efficiency with cascaded wiring of a stator winding.
Dynamoelectric machines in automotive applications include alternators, alternator-starters, traction motors, and others. The stator of an electric machine typically includes a cylindrical core formed as a stack of individual laminations and having a number of circumferentially spaced slots that extend axially through the stator core. A rotor assembly includes a center shaft and is coaxial with the stator core. The stator core has wires wound thereon in the form of windings that extend axially through ones of the core slots. End turns are formed in the windings at the two axial ends of the stator core, a given winding having an end loop as it extends circumferentially to a different slot. In this general manner, a stator winding extends axially from end to end in selected ones of the plurality of stator core slots and extends circumferentially between slots, according to a chosen wiring pattern.
The stator may be formed with any number of separate phase windings, such as three-phase, five-phase, six-phase, etc., and such determines the general wiring pattern to be implemented when winding the stator core. Since most applications emphasize reducing the size of the electric machine while improving efficiency, it is desirable to utilize the available slots in a manner that maximizes the filling of the stator core slots. High slot fill stators generally produce more electrical power with increased machine efficiency. Use of rectangular conductor wire may achieve a slot fill ratio of 75% or greater. Hairpin conductors are U-shaped solid wires having a substantially rectangular cross-sectional profile that are inserted into two slots at one axial end of the stator core and that are twisted and then welded to other hairpins at the other axial end of the stator core, as part of a phase winding. However, use of hairpin conductors may require a tradeoff between achieving a high slot fill ratio and reducing undesirable AC performance characteristics such as skin effect and others. Skin effect reduces the effective cross-sectional area of a conductor in a slot as the thickness of the conductor increases. Therefore, generally, the thickness of rectangular wires in a slot should be made as small as possible. Alternatively, a given wiring configuration may be designed to greatly reduce undesirable performance, for example by placing more than one phase in a slot.
Manufacturing problems and associated increased costs may also be encountered when forming and welding hairpin conductors. For example, connecting the ends of hairpins at one axial end of a stator typically requires a large number of welds. In addition, each hairpin may be required to be staggered or interleaved with respect to adjacent hairpin end portions, and the insertions, bending, and routing of individual hairpins necessitate a large number of manufacturing steps.